staging: comedi: comedi_fops: use pr_fmt()

There are a number of pr_{level} messages in this file that are used
to print kernel messages when the device pointer is not available for
a dev_{level} message.

Use pr_fmt() to ensure all of these messages have the module prefix.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>